1. Field of the Invention
The present invention relates to a phosphor in which luminescence generated by applying electric field (electro luminescence) and to a light emitting device using the phosphor. In particular, the present invention relates to a light emitting device in which an organic compound is used for a phosphor. Fluorescence and phosphorescence are included in the electro luminescence. The present invention relates to a light emitting device to which light emission by one or both of the fluorescence and phosphorescence is applied.
2. Description of the Related Art
A back light or a front light is used as the typical form of a display device using liquid crystal, and the structure is such that an image is displayed by means of the light. The liquid crystal display device is adopted as an image displaying means in various electronic devices, but has a defect from the viewpoint of its structure that a viewing angle is narrow. On the contrary, a display device using a phosphor in which electro luminescence is obtained has a wide viewing angle and is excellent in visibility. Thus, the display device using a phosphor has attracted attention as the display device in the next generation.
A light emitting element in which an organic compound is used for a phosphor (hereinafter referred to as organic light emitting element) is structured by appropriately combining a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer, and an electron injecting layer, which are formed by the organic compound, between a cathode and an anode. Here, the hole injecting layer and the hole transporting layer are separately shown, but these are the same in a meaning that a hole transporting property (hole mobility) is particularly an important characteristic. In order to distinguish these layers for convenience, the hole injecting layer indicates the layer that contacts the anode, and the layer that contacts the light emitting layer is referred to as the hole transporting layer. Further, the layer that contacts the cathode is referred to as the electron injecting layer, and the layer that contacts the light emitting layer is referred to as the electron transporting layer. The light emitting layer may also serve as the electron transporting layer, and thus, is also referred to as a light emitting electron transporting layer. The light emitting element formed by combining these layers shows a rectification characteristic, and has the same structure as a diode.
The light emitting mechanism is considered such that an electron injected from the cathode and a hole injected from the anode are recombined in a layer comprised of a phosphor (light emitting layer) to form an exciton, and the exciton emits light when returning to a base state. There are emission of light from a singlet excitation state (fluorescence) and emission of light from a triplet excitation state (phosphorescence). Luminance reaches several thousands to several tens of thousands cd/m2. Thus, it is considered that the light emission mechanism can be applied to a display device and the like in principle. However, various types of deterioration phenomena exist, and remain as a problem that impedes putting of the display device and the like to practical use.
The deterioration of the phosphor comprised of the organic compound or the organic light emitting element is considered to arise from the five factors mentioned below. The factors are (1) chemical deterioration of the organic compound (through the excitation state), (2) melting of the organic compound due to heat generation at the time of drive, (3) dielectric breakdown caused by a macro defect, (4) deterioration of an electrode or an interface of the electrode and an organic layer and (5) deterioration that arises from instability in an amorphous structure of the organic compound.
The above factors (1) to (3) are caused by driving the organic light emitting element. Heat is inevitably generated by such that a current in the element is converted into Joule heat. It is considered that melting occurs when the melting point or the glass transition temperature of the organic compound is low. Further, electric field is concentrated on the portion where a pin hole or a scratch exists, whereby the dielectric breakdown occurs. As to the factors (4) and (5), the deterioration progresses even if the phosphor comprised of the organic compound or organic light emitting element is preserved at a room temperature. The factor (4) is known as a dark spot, and arises from oxidization and reaction with moisture of a cathode. As to the factor (5), the organic compound used in the organic light emitting element is an amorphous material, and it is considered that almost no amorphous material exists in which the amorphous structure is kept stable since amorphous materials are crystallized by the long preservation, change through the elapse of time and heat generation.
The dark spot has been considerably suppressed because of the improvement of a sealing technique. However, the actual deterioration is occurred with the combination of the above factors, and thus, it is difficult to commonly understand the actual deterioration. The typical sealing technique is known as a method of making an organic light emitting element formed on a substrate airtight by a sealing member and providing a drying agent in the space. However, it is considered that the phenomenon, in which not only the current flowing through the organic light emitting element but also the emission luminance are towered when a constant voltage is continuously applied, originates in the property of the organic compound.
A low molecular weight organic compound and a polymer organic compound are both known as an organic compound for forming an organic light emitting element. As one example of the low molecular weight organic compound, copper phthalocyanine (CuPc), or xc3xa1-NPD (4,4xe2x80x2-bis-[N-(naphthyl)-N-phenyl-amino]biphenyl) or MTDATA (4,4xe2x80x2,4xe2x80x3-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine), which is an aromatic amine-based material is known as the hole injecting layer, and tris-8-quinolinolate-aluminum complex (Alq3) or the like is known as the light emitting layer. As the polymer organic light emitting material, polyaniline, polythiophene derivative (PEDOT) or the like is known.
It is considered that the low molecular weight organic compound formed by an evaporation method has remarkable variety in comparison with the polymer organic material from the viewpoint of the variety of materials. However, in any case, the organic compound constituted of only a basic structural unit is rare. There may be the cases where different kinds of the organic compounds are combined, an impurity is mixed into the organic compound in a manufacturing process, and various additives such as a pigment are added to the organic compound. Further, among these materials, a material deteriorated due to moisture, a material easily oxidized and the like are included. Moisture and oxygen can be easily mixed from an atmosphere. Thus, care needs to be taken in handling the materials.
It is known that chemical bond is changed into double bond and the structure containing oxygen (xe2x80x94OH, xe2x80x94OOH,  greater than Cxe2x95x90O, xe2x80x94COOH and the like) when the organic compound is subjected to light deterioration. Therefore, in the case where the organic compound is provided in the atmosphere containing oxygen, or in the case where oxygen or H2O as an impurity is included in the organic compound, it is considered that the bond state changes, and the deterioration is promoted.
In the field of a semiconductor technique, in a semiconductor element having semiconductor junction, such as a diode, an impurity that arises from oxygen forms a local level in a forbidden band, which is a cause of junction leakage reduction and lifetime of or carrier. Thus, it is known that the impurity remarkably reduces the characteristics of the semiconductor element.
Oxygen molecules are peculiar molecules in a base state and also in a triplet state since a highest occupied molecular orbital (HOMO) is in condensation polymerization. Generally, the excitation process from triplet to singlet is forbidden transition (spin forbidden), and thus, is hard to occur. Therefore, oxygen molecules in the singlet state are not generated. However, when the molecules in the triplet excitation state (3M*) that is a state with higher energy than that of the singlet state exist around the oxygen molecules, energy transfer such as the following occurs. Thus, the reaction in which the oxygen molecules in the singlet state are generated can be found.
3M*+3O2xe2x86x921M+O2xe2x80x83xe2x80x83Formula 1
It is said that 75% of the excitation state of molecules in a light emitting layer of an organic light emitting element corresponds to the triplet state. Therefore, in the case where oxygen molecules are mixed in the organic light emitting element, the oxygen molecules in the singlet state can be generated by the energy transfer in the formula 1. The oxygen molecules in the singlet excitation state have ion properties (there is polarization in electric charge). Thus, it is considered there is a possibility that the oxygen molecules react with the charge polarization generated in the organic compound.
For example, since a methyl group is electron donor in basocuproin (hereinafter referred to as BCP), carbon directly bonded to a conjugate ring is electrified in positive. As shown in the following chemical formula 1, singlet oxygen having ion properties reacts with oxygen molecules in positive electrification if the oxygen molecules exist. Thus, there is a chemical formula 2. As a result, it is expected that the electron transporting property is lowered. 
The present inventor has found that an impurity such as oxygen or H2O contained in an organic compound causes various types of deterioration such as reduction of luminance in an organic light emitting element and an organic light emitting device using the same based on the study described above.
In the organic light emitting element having the layer comprised of the organic compound between a cathode and an anode, and in the light emitting device structured using the organic light emitting element, it is necessary to reduce oxygen concentration that brings about the reduction of luminance and deterioration of an electrode material, such as a dark spot.
A preferred applied example using the organic light emitting element is an active matrix drive light emitting device, in which a pixel portion is formed in the organic light emitting element. A thin film transistor (hereinafter referred to as TFT) as an active element is provided in each pixel. However, it is known that characteristic values such as threshold voltage fluctuate due to contamination of an alkali metal with respect to the TFT formed using a semiconductor film. In the present invention, an appropriate structure for forming the pixel portion by combining the organic light emitting element, in which an alkali metal with a small working function is used in a cathode, and the TFT is required.
The active matrix drive light emitting device in which the pixel portion is formed by combining the organic light emitting element and the TFT is structured by appropriately combining a semiconductor material containing silicon as its main constituent and an inorganic or organic insulating material containing silicon as its constituent. The external quantum efficiency of the organic light emitting element does not still reach 50%. Thus, most of injected carriers are converted into heat, whereby the light emitting element is heated. As a result, thermal stress is applied to the light emitting element and acts on the respective layers forming a pixel. There occurs a defect that a crack is generated if the thermal stress is large.
The present invention has been made in view of the above and an object of the present invention is therefore to prevent deterioration that arises from chemical and physical factors in a light emitting device and to improve the reliability.
In order to prevent the deterioration of the light emitting device, the present invention is characterized in that an impurity containing oxygen, such as oxygen or H2O which is contained in an organic compound that forms an organic light emitting element is reduced. Of course, oxygen, hydrogen and the like are included as the structural elements of the organic compound. However, in the present invention, the impurity to the organic compound refers to an extrinsic impurity which is not included in the original molecular structure. Such an impurity is considered to be present in the organic compound as an atomic impurity, a molecular impurity, a free radical or an oligomer.
Further, according to the present invention, in the active matrix drive light emitting device, the structure for preventing fluctuation of a threshold voltage due to contamination of a TFT by an alkali metal such as sodium or potassium is provided.
According to the present invention, such an impurity is removed, and the impurity concentration of the layers formed of the organic compound that is used for forming the organic light emitting element, such as a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer and an electron injecting layer is reduced to 5xc3x971019 atoms/cm3 or less, preferably, 1xc3x971019 atoms/cm3 or less as the average concentration. In particular, it is required that the oxygen concentration in the light emitting layer and in the vicinity thereof is reduced.
When the organic light emitting element emits light with luminance of 1000 Cd/cm2, if this is converted into photons, the resultant corresponds to an emission amount of 1016 photons/see cm2. Assuming that the quantum efficiency of the organic light emitting element is 1%, a current density of 100 mA/cm2 is required. In accordance with an empirical rule based on a semiconductor element such as a solar battery or a photo diode using an amorphous semiconductor, the defect level density needs to be set to 1016 atoms/cm2 or less in order to obtain satisfactory characteristics in the element through which the above current flows. For realizing the value, it is required that the concentration of a vicious impurity element forming the defect level is reduced to 5xc3x971019 atoms/cm3 or less, preferably, 1xc3x971019 atoms/cm3 or less as described above.
In order to reduce the impurity of the organic compound forming the organic light emitting element, a manufacturing device for forming the organic light emitting element comprises the following structure.
In an evaporation device for forming a layer comprised of a low molecular weight organic compound, wall surfaces of the inside of a reaction chamber is made specular by electropolishing to reduce a gas emission amount. Stainless steel or aluminum is used as the material for the reaction chamber. For the purpose of preventing the gas emission from the inner walls, a heater is provided outside the reaction chamber and a baking process is conducted. The gas emission can be considerably reduced by the baking process. Further, cooling is preferably conducted by means of a coolant at the time of evaporation. A turbo molecular pump and a dry pump are used for an exhaust system, and reverse diffusion of oil vapor from the exhaust system is prevented. Moreover, a cryopump may be provided in order to remove remaining H2O.
An evaporation source is basically resistance heating type, but Knudsen cell may be used. A material for evaporation is brought into from a load lock type exchanging chamber attendant on the reaction chamber. Thus, the exposure to an atmosphere of the reaction chamber is avoided as much as possible at the time of loading the material for evaporation. The evaporation source is mainly comprised of the organic material, and the purification by sublimation is performed at the inside of the reaction chamber before evaporation. In addition, a zone refining method may be applied.
With respect to the pre-processing of a substrate to be introduced into the reaction chamber, gas emission processing by heating and plasma processing using argon are conducted, and thus, the impurity emitted from the substrate is reduced as much as possible. In the active matrix drive light emitting device, a TFT is formed in advance on the substrate on which the organic light emitting element is to be formed. In the case where an insulating layer, etc. using an organic resin material are appropriately used as the structural components of the substrate, it is necessary to reduce gas emission from the member. Further, nitrogen gas or argon gas, which is introduced into the reaction chamber, is refined at a supply port.
On the other hand, in the case where a layer comprised of a polymer organic compound is formed, control on degree of polymerization cannot be completely conducted, and thus, a range of molecular weight develops. Therefore, the melting point may not be determined with only one meaning. A dialysis or high-performance liquid chromatography is appropriate for such a case. Particularly, in the dialysis, an electrodialysis is suitable for efficiently removing an ion impurity.
In an active matrix drive method in which a pixel portion is formed by an organic light emitting element formed as described above and each pixel is controlled by an active element, one embodiment of the structure is such that a TFT having a semiconductor film, a gate insulating film and a gate electrode is formed on a substrate and an organic light emitting element is formed above the TFT. A typified example of the substrate to be used is a glass substrate, and a small amount of alkali metal is contained in barium borosilicate glass or alumino borosilicate glass. The semiconductor film is coated by silicon nitride or silicon oxynitride in order to prevent contamination due to the alkali metal from the glass substrate on the lower layer side and the organic light emitting element on the upper layer side.
On the other hand, the organic light emitting element that is desirably formed on the leveled surface is formed on a leveling film comprised of an organic resin material such as polyimide or acrylic. However, such an organic resin material has hygroscopic property. The organic light emitting element that is deteriorated by oxygen or H2O is coated with silicon nitride, silicon oxynitride or diamond-like carbon (DLC) having a gas-barrier property.
FIG. 12 is a diagram explaining the concept of the active matrix drive light emitting device according to the present invention. As the structural components of a tight emitting device 1200, a TFT 1201 and an organic light emitting element 1202 are formed on the same substrate. The structural components of the TFT 1201 are a semiconductor film, a gate insulating film, a gate electrode and the like, and the elements contained in those are silicon. hydrogen, oxygen, nitrogen, metal forming a gate electrode, and the like. On the other hand, the organic light emitting element 1202 contains an alkali metal such as lithium as the element in addition to carbon that is a main structural component of an organic compound material.
Silicon nitride or silicon oxynitride 1205 is formed as a blocking layer on the lower layer side of the TFT 1201 (glass substrate 1203 side). Silicon oxynitride 1206 is formed as a protective film on the opposite and upper layer side. On the other hand, silicon nitride or silicon oxynitride 1207 is formed as a protective film on the lower layer side of the organic light emitting element 1202. As the protective film, aluminum oxide, aluminum nitride, aluminum oxynitride can also be applied. A DLC film 1208 is formed as a protective film on the upper layer side.
An organic resin interlayer insulating film 1204 is formed between the TFT and the organic light emitting element and is integrated therewith. The alkali metal such as sodium that most easily affects the TFT 1201 is blocked by the silicon nitride or silicon oxynitride 1205 or the silicon oxynitride 1206. On the other hand, since the organic light emitting element 1202 most dislikes oxygen or H2O, the silicon nitride or silicon oxynitride 1207 and the DLC film 1208 are formed in order to block oxygen or H2O. Further, the silicon nitride or silicon oxynitride 1207 and the DLC film 1208 have functions of not letting the alkali metal element of the organic light emitting element 1202 out.
As described above, the light emitting device structured by combining the TFT with the organic light emitting element is formed by cleverly combining the insulating films having a blocking property against oxygen or H2O in order to satisfy opposite properties with respect to impurity contamination.
With the above-described structural components as the basis, the light emitting element having an anode, an organic compound layer, and a cathode containing an alkali metal is formed between partition layers comprised of an insulating material. The partition layer has a shape in which the upper portion protrudes in a direction parallel to the substrate (so-called overhang shape) and takes a structure in which the organic compound layer and the cathode layer of the organic light emitting element do not contact with each other.
The organic compound material forming the light emitting element is refined, the impurity mixing is prevented in the film formation, and the organic compound layer is highly purified, whereby the reduction of luminance and the deterioration of the cathode layer can be prevented. Further, an inorganic insulating layer comprised of silicon nitride or silicon oxynitride or the like is provided between the light emitting element and the TFT, and thus, the alkali metal element forming the cathode layer can be prevented from diffusing into the semiconductor film that constitutes the TFT. In the light emitting element, the partition layer has the overhang shape, and the structure is taken in which the organic compound layer and the cathode layer of the organic light emitting element do not contact with each other. Thus, thermal stress acts on the respective layers that constitute the light emitting device with the thermal stress, and the generation of physical damage such as a crack can be prevented. With the above action, the reliability of the light emitting device can be enhanced.
Note that the light emitting device throughout this specification indicates the whole devices using the phosphor. Further, a module in which an element having a layer comprising the phosphor between an anode and a cathode (hereinafter referred to as light emitting element) is attached with a TAB (tape automated bonding) tape or a TCP (tape carrier package), a module in which a printed wiring board is attached to the end of the TAB tape or the TCP, or a module in which an IC is mounted to the substrate, on which the light emitting element is formed, by a COG (chip on glass) method are all included in the light emitting devices.
Further, the concentration of oxygen as the impurity element in this specification indicates the minimum concentration measured by a secondary ion mass spectrometry (SIMS).